Mutations conferring resistance to 8-azaguanine or to 2,6- diaminopurine occur spontaneously in cultures of diploid human fibroblasts and can be induced with X-rays and MNNG. The aim of this project is to analyze the nature of the mutations using genetic and biochemical approaches, such as somatic cell hybridization and characterization of enzymes. Some may occur in structural genes, others in regulatory genes while others epigenetically. Diaminopurine- resistance often results from a deficiency of the enzyme A-PRT, which is specified by genes on autosome 16. Therefore, phenotypic mutants most probably originate in heterozygous cultures and may result from gene mutation, deletion, chromosome loss or mitotic recombination. The hypothesis that cultures yielding diaminopurine-resistant mutants are heterozygous is being tested by attempting to achieve segregation of the mutant and normal alleles into separate hybrids formed with mouse cells. Mutagenesis is used to create fibroblasts strains that have mutations and chromosome aberrations, which make it possible to detect derepression of the inactive X-chromosome. The fusion of such human female cells with mouse cells has produced evidence for possible derepression of the X in hybrid cells. The objective is to learn about the mechanism of repression.